The overall goal of Project 2 will be to characterize the reconstitution of regulatory T cells after allogeneic HSCT and to define the role of these important regulatory elements in the development of GVHD. Recent studies from many laboratories using a variety of model systems have demonstrated that regulatory T cells (Treg) play an important role in the suppression of autoimmunity and that deficiencies of Treg are capable of enhancing tumor immunity as well as autoimmunity. Several laboratories have also begun to examine the potential role of Treg in the development of GVHD following allogeneic HSCT in humans. Unfortunately, initial studies have provided conflicting results and the role of Treg in the modulation of GVL and GVHD is uncertain. Preliminary results from our laboratory suggest that deficient reconstitution of Treg is, in fact, associated with the development of chronic GVHD. In a comprehensive prospective analysis of patients undergoing allogeneic HSCT, our studies will determine whether Treg play a significant role in the modulation of T cell reconstitution in the allogeneic stem cell recipient. Using both phenotypic and functional assays, these studies will determine whether deficient reconstitution of Treg contributes to the development and persistence of chronic GVHD. Further studies will also examine mechanisms affecting the reconstitution of Treg and potential methods for modulating Treg in vivo. We also propose to initiate clinical trials of adoptive therapy with donor Treg that have been isolated and expanded in vitro. Working with other investigators in this Program Project, these experiments will lead to the development of new methods for selectively enhancing beneficial responses or selectively abrogating toxic responses after allogeneic HSCT. These experiments will be carried out in 4 Specific Aims: 1. To determine whether chronic GVHD is associated with delayed reconstitution of regulatory T cells after allogeneic HSCT. 2. To define the mechanisms of reconstitution of regulatory T cells after allogeneic HSCT. 3. To define mechanisms for modulation of regulatory T cells in vivo after allogeneic HSCT. 4. To evaluate the toxicity and immunologic effects of adoptive cellular therapy with regulatory T cells.